1. Field of the Invention
The present invention relates to sheet feeding apparatus and more specifically to sheet feeding apparatus for insuring that only the top sheet of a stack of sheets, such as labels and box wrapper sheets will be fed from the apparatus to a gluing machine, or to any other processing machine, without disturbing the underlying sheets of the stack.
2. Description of the Prior Art
It has been found that vacuum pickup heads at the front of the stack may cause the feeding of more than the top sheet in a stack. For example, paper sheets are commonly formed by shearing or cutting a stack of sheets at once. If the knives are dull, this causes the paper fibers along the sheared sides of the stack to interlock to some degree so that if attempts are made to merely slide the top sheet across the second sheet, the second sheet may be pulled along with the top sheet. A similar problem arises when the sheets are embossed; such sheets resist pure sliding separation.
A prior art sheet feeding device of the type to which one embodiment of the present invention relates is disclosed in the Nitsch et al U.S. Pat. No. 1,684,741, issued on Sept. 18, 1928. The U.S. Pat. No. 2,726,861, to Wolff et al issued Dec. 15, 1955, discloses a vacuumized rear sheet separator over which the rear sheet separator of the present invention is an improvement.
The feeders of these patents are associated with gluing machines, wherein the top sheet of a stack of sheets is fed into glue rollers. Both patented structures employ elevating tables which support the stack of sheets, and which are incrementally elevated to maintain the uppermost sheet near a given pickup position.
In the sheet feeding mechanism of the Nitsch et al U.S. Pat. No. 1,684,741, an articulated front vacuum pickup head overlies the front edge portion of the stack. The rear edge of the stack is engaged by a slidably mounted foot which holds the sheets down by force of gravity which mounts a back stop. When a sheet is to be removed from the stack, vacuum is applied to the front pick up head, and the head with a sheet gripped thereby is lifted to bring the front edge of the sheet against an upper, driven feed roll. A lower feed roll is then brought against the sheet and the feed rolls slide the sheet across the stack for transport past glue rolls and onto a delivery conveyor. In this patented device, although the front end of the sheet is lifted by the front vacuum feed head, the feed head itself does not slide the top sheet along the second sheet while the top sheet is being lifted, all such sliding action results solely from the grip of the feed rolls on the sheet after the front edge of the sheet has been lifted by the front vacuum head.
In the Wolff et al U.S. Pat. No. 2,726,861 the simple hold down device of the Nitsch et al patent is replaced by a combined vacuumized rear sheet separator and a mechanically actuated hold down foot. Here the rear of the top sheet in the stack is lifted by a pivoted, flat faced vacuum head, whereupon a mechanically actuated finger or hold down foot holds down the second sheet. A vacuum feeding head at the front of the stack grips the front edge of the top sheet, and not only lifts the sheet, but slides it across the stack and between the feed rolls. While the front vacuum head is thus feeding the sheet, the vacuum to the rear sheet separator is cut off. However, the rear hold down foot engages the stack and thus prevents the front vacuum head from pulling the second sheet along with the top sheet. A rearwardly directed air jet is provided at the front of the stack.
Referring to the construction and mode of operation of the rear sheet separator itself, of the Wolff et at patent, the rear vacuum or suction head is pivotally mounted in the chamber of a floating block. The vacuum line connects to the chamber in the block, and not directly to the rear suction head. The suction head is spring biased downwardly to contact a sheet. Apertures extending through the suction head communicate with the chamber, and it is intended that these apertures be closed off when the suction head is in contact with the top sheet. When vacuum is applied to the chamber, the differential or net atmosphere pressure acting on the underside of the top sheet and hence on the suction head, must lift the weight of the head against the bias of the spring, and must overcome friction forces developed by any sealing contact that exists between the suction head and the walls of the vacuum chamber in which the suction head is mounted. Of course, this means that leakage of air around the suction head reduces the effective or differential pressure available to lift the suction head and the associated sheet. Furthermore, if the sheets are somewhat porous or pervious to air, and if the degree of vacuum applied to the chamber is adequate to lift the suction head against its own weight, against friction and against the force of the spring, such a vacuum can induce air flow through the uppermost sheet in the stack and into the vacuum ports in the suction head. Air flow through the top sheet can create a sub-atmosphere pressure zone between the top sheet and the second sheet below it, (Bernoulli effect), and may cause the rear edge of the second sheet to be picked up with the top sheet. Another characteristic of the device of Wolff et al is that the pivoted suction head and the walls of the chamber in the block within which it operates are subject to wear and require careful machining to minimize leakage.
A back stop is required in devices of this type. In the device of Nitsch el at, the back stop is mounted on the hold down foot and will be lifted by the feed table before a full stack has been removed. The back stop of Wolff et al, is mounted on a vertically stationary block that floatingly supports the rear sheet separator assembly, so that the back stop limits the lift of the feed table.